1. Field of the Invention
The present invention relates to magnetron control circuits for use in powering a magnetron in a microwave oven.
2. Description of the Prior Art
In conventional microwave ovens, microwaves are generated by a magnetron which is triggered for repetitive portions of an a.c. power signal cycle under the control of a large triac. The triac is gated by a control signal, typically emanating from a microprocessor controller.
It is highly desirable for the magnetron control circuitry to be electrically isolated from the microprocessor. Because of the high voltages and currents present in the magnetron control system, any discharge therefrom back through the control circuitry into the microprocessor controller can result in irreparable damage to the expensive microprocessor controller, or in injury to the oven operator. Unless precautions are taken, the oven operator can receive an electrical shock from the oven temperature probe, time and temperature dials, and other low voltage operating controls in the system. In the event of a malfunction, the absence of electrical isolation of the magnetron can result in a voltage and current feedback through the microprocessor which posses a threat of serious energy to persons coming in contact with the otherwise harmless controls. Accordingly, electrical isolation of the magnetron power circuit is a highly desirable feature.
In the past, various techniques have been employed to electrically isolate the magnetron power circuitry from operating controls which one might touch. One such technique is to employ a light emitting diode at the control signal line output of the microprocessor controller and a corresponding light senstive diode coupled to the magnetron power circuit. Both the light emitting and the light sensitive diodes are located in a darkened enclosure. Such an arrangement establishes the necessary electrical isolation of the magnetron power circuit. However, such a circuit is relatively expensive and requires considerable space in a circuit enclosure which is quite limited as to availability of space. Specifically, a housing to provide the necessary shielding for the light sensitive and light emitting diodes is necessarily voluminous, compared to the other solid state components on the printed circuit oven control board, and represents a signficant sacrifice in available space in a circuit enclosure which is typically only a few cubic inches in volume.
Other types of electrical isolation include transformer circuits in which voltage and current levels are closely regulated on the primary leads. This prevents inordinately large voltage and current feedback in instances of malfunctions. However, such an arrangement as heretofore involved unnecessary complexity in the microprocessor program. In order to provide an actuating signal. A timed output from the microprocessor is typically required so as to trigger an inductive output from a control transformer. This requires gating through an oscillator signal from one of the master oscillators of the microprocessor control system and a recurring servicing program routine that occupies the microprocessor to the neglect of other control functions.
Another prior art arrangement that avoids extensive programming utilizes a relaxation oscillator circuit as a substitute for program controlled gating pulse timing. However, such arrangements are quite slow and do not begin delivering regular gating pulses smoothly until after a certain settling period.